An optical information recording medium represented by a compact disc (CD), a Digital Versatile Disc (DVD), and a Blu-ray® Disc (BD) increases the recording density by mainly shortening the wavelength of a laser beam and increasing the numerical aperture (NA) of an object lens. It is, however, considered that a limit has almost been reached for technical reasons, and thus it is desirable to increase the recording density by other methods or schemes.
There have been developed a high-density volumetric optical recording memory (to be referred to as a holographic memory hereinafter) using holography, and a recording and reproducing apparatus for a holographic memory. Unlike a so-called surface recording method of recording a recording mark on a recording surface, such as a DVD, a holographic optical disk used for recording of a holographic memory uses a volumetric recording method with which it is possible to record data in the depth direction of an information recording layer, and thus a high recording density is obtained as compared with a DVD or the like.
For a DVD or the like, the recording mark generally represents on/off bit data. In contrast, for a holographic memory, a relatively large amount of information is modulated at once, and is recorded as interference fringes in a recording medium. The set of information is a modulation pattern for information light held in the recording medium, and is a smallest unit of a two-dimensional barcode with white and black dots for recording and reproduction, which is called a reproduction image (page data). Since it is possible to acquire a large amount of information by one recording or reproduction operation, a high transfer rate can be achieved.